Investments in Partially Owned Entities (Urban Edge Properties and PREIT Associates L.P.) (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 29, 2017
Schedule of Equity Method Investments [Line Items]
Non cash impairment losses		$ 44,465	$ 20,290	$ 21,260
Urban Edge Properties
Schedule of Equity Method Investments [Line Items]
Investment owned, balance, shares		5,717,184
Equity method ownership percentage		4.50%	5.40%
Net gain resulting from UE operating partnership unit issuances		$ 21,100	$ 0	$ 0
Closing share price (in dollars per share)		$ 25.49
Equity method investment, quoted market value		$ 145,731
Excess of investee's fair value over carrying amount		$ 99,579
PREIT
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		8.00%
Closing share price (in dollars per share)		$ 11.89			$ 10.49
Equity method investment, quoted market value		$ 74,313
Excess of investee's fair value over carrying amount		$ 7,741
Operating partnership units received from buyer		6,250,000
Amount of fair value in excess of carrying amount	$ 44,465
Non cash impairment losses	44,465
Excess of investee's carrying amount over equity in net assets		$ 34,205
Fair Value, Measurements, Nonrecurring | Level 1 | PREIT
Schedule of Equity Method Investments [Line Items]
Equity method investment, quoted market value	$ 65,563
Partnership Interest | Urban Edge Properties
Schedule of Equity Method Investments [Line Items]
Operating partnership capital account units issued		20,250,000